Improves Merge() operation by also pruning old key/value pairs (#29)

* Added new API Stats() and Prune()

* Improved Merge() logic to also prune old key/values and actually reclaim disk space

* Added backward compat for the old Merge() function

* Refactor indexing of keys to items (hints)

* Remove redundant TestOpenMerge

* Add unit test for Stats()

* Improve TestMerge()

internal/keydir.go

@@ -5,6 +5,7 @@ import (
 	"encoding/gob"
 	"io"
 	"io/ioutil"
+	"os"
 	"sync"
 )
 
@@ -81,6 +82,21 @@ func (k *Keydir) Bytes() ([]byte, error) {
 	return buf.Bytes(), nil
 }
 
+func (k *Keydir) Load(fn string) error {
+	f, err := os.Open(fn)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	dec := gob.NewDecoder(f)
+	if err := dec.Decode(&k.kv); err != nil {
+		return err
+	}
+
+	return nil
+}
+
 func (k *Keydir) Save(fn string) error {
 	data, err := k.Bytes()
 	if err != nil {